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Organic Fertilizers in a Soilless Growing System

Posted by Jim Brown, Horticulturist on 9/9/2006

Organic growing can be accomplished via either soil-based or lower-volume soiless systems. Soiless organic growing systems have been discussed in detail in previous articles. However, as a brief synopsis, soiless systems use materials such as perlite, pine bark, coconut core, compost and sawdust as a growing media, just to name a few, and these systems are usually found in a greenhouse growing environment. As in all growing systems, proper fertilization of the plants is key to healthy and vigorous growth and optimal production. In the case of a soiless media system, the type of media used plays a role in determining what fertilization is needed. The microorganism present in the media have the job of breaking down the various fertilizer components so that they are readily absorbed through the plant roots There are some challenges to supplying fertility in organic low-media-volume production systems.

In organic systems, certain fertilizer ingredients are more difficult to supply to the plants. The fertilizer source forms, the media and the microorganism decomposition needed for proper fertilization will be briefly discussed in this article, with a focus on the fertilizer components that present the greatest challenge. Some related issues with fertilizer carriers and media buildup are also included.

Plants grow by accumulating and organizing substances from outside themselves. Those substances come into the plant in the air that is absorbed into the leaves or in the water taken up by its roots. Although some supplementation of the plant through its leaves can be accomplished, we will just look at fertilization that can be supplied to the plant roots through the media surrounding them.


Nitrogen is an element needed for plant growth and development. It is an essential component of the amino acids and proteins in the plant. However, not just any source of nitrogen is able to be absorbed by the plant. Although the air around us is approximately eighty per cent nitrogen, the nitrogen contained in the air is not available to the plant.

Nitrogen can be supplied to plants growing in a media in three to four different molecular or ionic forms. Those include urea, ammonia, nitrite and nitrate. Nitrogen-containing molecules are very water soluble. When present in the plant root environment, they can be easily taken up and utilized by the plant. Nitrogen-containing molecules or ions that are not taken up by plants or micro organisms within the media can be washed away by moving water through the media. Nitrogen does not remain in the plant media in a soluble form for very long. It is either taken up by plants, by micro organisms in the media or it is washed away in media drainage.

Urea and ammonia are breakdown products of proteins and amino acids. One or both are found in the waste products of animals and are either a byproduct of protein that has been broken down during the digestive process or the result of the breakdown of proteins or amino acids from cells that have been replaced or discarded by the body. Urea and ammonia can not be tolerated in large quantities by many plants. In excess, they will contribute to excessive vegetative growth in the plants. Many of the commercially available packaged fertilizers will utilize urea or ammonia nitrogen forms. The amount that can be included and supplied at one time is limited because of potential plant damage.

Therefore, it is usually desirable to have these nitrogen forms oxidized to the nitrate nitrogen form. How does oxidization occur?

Many of the microorganisms present in organic growing media utilize urea or ammonia as an energy source. They oxidize these substances into nitrite and then to nitrate. The nitrite form is an intermediate oxidation state that usually quickly progresses to the nitrate form. Different microorganisms are often involved in each of the two processes. Both types of microorganisms are usually present in a plant media system. Compost and compost teas are frequently used as sources of the microorganisms needed to boost the microbial activity in the plant production media.

Nitrogen in all its plant-available forms can be easily washed away from the media in which the plants are growing. A continuous re-supply of nitrogen, therefore, is needed for continuing plant growth and development.

Some plant residue, especially green manure crops, can provide a relatively long lasting supply of plant-available nitrogen. However, such crop residue requires more nitrogen for its microbial decomposition than it supplies. Although the nitrogen processed by the microbes is eventually released and becomes available to the plants, the plants do not receive it until the microbes have completed the decomposition process. Providing the correct balance of decomposable media and nitrogen is a balancing act that needs to be maintained for continuous adequate nitrogen availability to the growing plants in the media.


Calcium is used extensively in the plant as a component of mature cell walls. As long as the plant is growing, it needs a steady supply of calcium for proper growth and development.

The only soluble calcium compounds are lime and calcium chloride. The amount of lime that can be used as a fertilizer is limited because it drives the pH up and interferes with the operation of the microbial population in the media. Calcium chloride almost must be used in small quantities, since the accumulation of the chloride ion interferes with the uptake of other negatively charged fertilizer ions and is toxic to the microbial population.

Calcium sulfate is used in organic plant production systems as a calcium source. Calcium sulfate is essentially insoluble in water and is not available to the plants in its basic form. It can be finely ground and suspended in water for distribution within the production system, where it is processed by the microbial population in the media before the calcium is released into the media in a plant-available form. The processing takes time and the microbial system must remain in balance with its other needs for a maximum amount of calcium to be made available to the plants.

Calcium sulfate also provide sulfur to the plants, which need some sulfur for adequate growth. Far more sulfate will be provided to the organic plant production system than will be needed by the plants. The plants and the microbial system in an organic production system are fairly tolerant to the presence of high levels of sulfate. There is a limit, however, on how much sulfate the system can tolerate before it starts interfering with the operation of the system. In the field and in growing systems where soil is used, the per-plant ratio of media to plant is much higher than it is in the greenhouse and in containerized organic production systems. Increasing the volume of media in the system increases the system’s holding capacity for cumulative substances like sulfate.


Water must be supplied to the “organic” plant production system. When this water is well water, river water or reservoir water, an accumulation of minerals may be present in it. The source water should be analyzed to determine its mineral content. Many of the ingredients will be usable by the plants.

The presence in the water of very soluble substances like sodium and chloride are a concern. Although plants use and need small amounts of both sodium and chloride, a very small amount will suffice. Many times there is much more present than what is needed.

When the water contains significant amounts of sodium and chloride, fertilizer ingredients should be selected that have little or no sodium or chloride in them. The larger the volume of water run through the organic production system, the greater the amount of sodium and chloride that will be removed from the system. When the volume of water flushing through the system is limited, the sodium and chloride content in the media will build faster.

High levels of either sodium or chloride in the plant production system will interfere with the proper plant nutrient uptake. Higher levels of sodium and/or chloride accumulation in the media can also interfere with adequate water uptake by the plants.


Different plants have different tolerance levels for sodium and/or chloride levels as well as for soluble fertilizer levels in the media.

Generally speaking, the organic matter making up the growing media is a buffer for sodium and chloride. In other words, more sodium and/or chloride can be present in a plant production system having high levels of organic ingredients than in a mineral soil system or in any hydroponic system. Some media ingredients such as composted pine bark will introduce more sodium and chloride tolerance into the media system than will other ingredients. It is, therefore, hard to quote generalized content figures that are acceptable or too high. Different systems comprised of different media types will have different tolerance levels to these substances.

The purpose of the information provided in the discussion above is to outline how organic plant production systems work. The limiting factors are discussed in such a way as to alert you to what can shut down the growth of the plants in the system. There are, however, variables, such as the specific content of the media itself, that make it impossible to quote specific figures for thresholds.

As you set up or continue to operate your organic plant production system, there are some things you should do and some measurements you should make to have the information that will help you figure out what changes are needed if your system does not work or if it stops working.

A water analysis of the source water used in the production system is something that should be done before setting up the system. Check the levels of sodium, chloride and sulfate in the water. The higher they are, the faster they will accumulate in the production system.

Find out as much as you can about the fertilizers you are using in your system. If you are using a commercially available fertilizer mix, this can be difficult. Often the specific levels of sodium and sulfate are not stated. If you do not have a working knowledge of the chemistry involved, you may not understand all you find.

Have periodic media and tissue analyses conducted so as to monitor any buildup of sodium, chloride, or sulfate. You will also be able to monitor the usable nutrients in the media and in the plant tissue.

Balancing the factors affecting proper fertilization in an organic soiless growing system may require some initial tweaking, but the rewards and convenience of this type of system are well worth the efforts.

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